1. Field of the Invention
The present invention relates to a copier, printer, facsimile apparatus or similar image forming apparatus and a process cartridge for use in the same.
2. Description of the Background Art
An image forming apparatus of the type using an electrostatic image transfer system is conventional and configured to form an electric field between a photoconductive drum or similar image carrier and an intermediate image transfer body, sheet conveyor or similar moving member for thereby transferring a toner image formed on the image carrier. In this type of image forming apparatus, some toner is left on the image carrier after the transfer of the toner image to a subject body, e.g., the intermediate image transfer body or a sheet or recording medium. If part of the image carrier on which such residual toner is present is subject to the next image formation, then irregular charging or similar defective charging occurs on the above part of the image carrier and lowers image quality. It is a common practice to remove the residual toner from the image carrier with a cleaning device facing the surface portion of the drum between an image transfer position and a charge position.
The problem with the cleaning device mentioned above is that it needs an extra space for accommodating a waste toner tank configured to store the residual toner collected from the image carrier and a recycling path along which the residual toner is conveyed to be reused, making the entire apparatus bulky. Particularly, a current trend in the imaging art is toward a tandem image forming apparatus that assigns a particular image carrier to each color in order to meet the increasing demand for high-speed color image formation. If the cleaning device is applied to this kind of image forming apparatus, then a particular cleaning device must be assigned to each of a plurality of image carriers, making the above problem more serious.
To solve the problem stated above, Japanese Patent No. 3,091,323, for example, discloses an image forming apparatus using a simultaneous developing and cleaning system that causes a developing device to collect the residual toner. More specifically, the developing device, originally expected to develop a latent image, is used as cleaning means at the same time, so that a particular cleaning device does not have to be assigned to each image carrier. This contributes a great deal to the size reduction of the apparatus.
On the other hand, while a blade type of cleaning device configured to clean the surface of the image carrier with a cleaning blade is predominant today, a bladeless type of cleaning device is also extensively used. The bladeless type of cleaning device may use a brush roller for collecting the residual toner or a bias applying member for electrostatically collecting the residual toner. Further, a simultaneous developing and cleaning system configured to collect residual toner left on the surface of an image carrier with a developing device is known in the art, as taught in Japanese Patent No. 3,091,323.
The bladeless type of cleaning system, which rubs the image carrier more softly than the blade type of cleaning system, successfully extends the life of the image carrier. In addition, load exerted by the bladeless type of cleaning system on the image carrier is lighter than load exerted by the blade type of cleaning system, reducing drive load to act on a driveline assigned to the image carrier.
The simultaneous developing and cleaning system does not need the cleaning device because the developing device, originally not used for the purpose of cleaning, plays the role of cleaning means at the same time. The simultaneous developing and cleaning system is therefore advantageous in that it reduces the overall size of the apparatus.
Japanese Patent mentioned above further teaches a charging device for the above image forming apparatus that includes a charge roller held in contact with the image carrier for uniformly charging the image carrier. Conventional systems for uniformly charging an image carrier are generally classified into a contact or vicinity type of charging system using a charge roller or similar charging member contacting or adjoining the image carrier and a non-contact type of charging system using a corona charger or similar charger. The non-contact type of charging system has a problem that it produces ozone, NOx (nitrogen oxides) and other discharge products, which are undesirable from the environment standpoint. In this respect, the contact or vicinity type of charging system, which produces a minimum of discharge products, is superior to the contact or vicinity type of charging system. Presumably, therefore, the apparatus taught in the above document promotes both of the size reduction of the apparatus and the reduction of discharge products.
However, the apparatus, using the simultaneous developing and cleaning system and contact or vicinity type of charging system has the following problem left unsolved. Before the residual toner present on the image carrier is conveyed to a developing zone, it contacts and deposits on the charging member, obstructing uniform charging. This prevents the charging member from charging the surface of the image carrier to an expected potential or causes irregular charging or similar defective charging to occur, resulting in short image density, background contamination and other defects. This problem is not particular to the apparatus using the simultaneous developing and cleaning system, but arises so long as the residual toner is conveyed to a position where the image carrier and charging member contact each other without being removed from the image carrier.
Pending Japanese Patent Application No. 2002-254142 discloses an image forming apparatus configured to solve the problem stated above. The apparatus taught in this document includes a brush member or similar temporary holding means for collecting and temporarily holding, among toner grains left on an image carrier after image transfer, toner grains charged to polarity opposite to toner grains of regular polarity, which is identical with the polarity of a charge bias, thereby preventing the toner grains of opposite polarity from depositing on a charging member. Subsequently, the temporary holding means returns the above toner grains to the image carrier at preselected timing between consecutive image formation. The toner grains thus returned to the image carrier are collected by a developing device or transferred to a subject body of image transfer or a member for conveying the subject body.
In the apparatus described above, when the toner grains returned to the image carrier pass a charging zone, a charge bias is interrupted or a charging member is released from the image carrier, preventing the toner grains from depositing on the charging member. On the other hand, the toner grains of negative or regular polarity, also included in the residual toner grains on the image carrier, remain on the image carrier without being transferred to the charging member. In addition, the toner grains of regular polarity are conveyed to a developing zone during the next image forming step and therefore collected by carrier grains included in a developer and contribute to development. It follows that the toner grains of regular polarity do not adversely effect the image forming step.
In the case where the charge bias and toner grains of regular polarity are different in polarity from each other, the toner grains of regular polarity will bring about the problem stated earlier.
Further, the apparatus stated above does not need a blade type of cleaning device, as distinguished from a blade type of cleaning device. More specifically, a brush member, serving as the temporary holding means rubs the surface of the image carrier in place of a cleaning blade and has therefore the advantages stated previously.
However, the bladeless type of cleaning system has a problem to be described hereinafter. Silica, zinc separate and other additives contained in toner grains sometimes part from the toner grains due to, e.g., mechanical stresses acting during image formation. If such additives parted from the toner grains are pressed against the image carrier by a developer in a developing zone or by the brush member over a long time, then the additives adhere to the image carrier in the form of a thin film. This phenomenon is generally referred to as filming. Filming weakens the adhesion of the toner grains to the image carrier and thereby blurs or otherwise disfigures an image.
The additives, forming the film on the image carrier and electrically neutralized, cannot be electrostatically removed, but can be mechanically removed, as determined by experiments. The blade type of cleaning system can therefore shave off the additives from the image carrier, thereby solving the problems ascribable to filming. However, the bladeless type of cleaning system rubs the image carrier with a weaker force than the blade type of cleaning system, as stated earlier, and therefore cannot sufficiently shave off the film.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 8-137198, 8-137205, 9-211979, 11-190931, 2000-194242, 2000-242152, 2001-75448, 2001-117317 and 2001-356614.